1. Field of the Invention
This invention relates to the PTC conductive polymer compositions and electrical devices containing them, especially self-limiting strip heaters.
2. Summary of the Prior Art
PTC conductive polymer compositions are known for use in self-limiting strip heaters and in other electrical devices; such compositions can contain two crystalline polymers having substantially different melting points. It is also known to anneal PTC compositions, after they have been shaped, in order to reduce their resistivity, by heating them for extended period, e.g. of several hours, at a temperature above the melting point of the composition. Reference may be made for example to U.S. Pat. Nos. 3,793,716, 3,823,217, (Kampe), 3,861,029 (Smith-Johannsen et al), U.S. Pat. No. 3,914,363 (Bedard et al), U.S. Pat. No. 4,177,376 (Horsma et al) and to U.S. patent application Ser. Nos. 84,352 filed 10-12-79 (Horsma et al), 732,792 filed 10-15-76 (Van Konynenburg et al) now abandoned, 751,095 filed 7-2-85 (Toy et al), 798,154 filed 5-18-77 (Horsma), now abandoned, 965,343 filed 12-1-78 (Van Konynenburg et al) now U.S. Pat. No. 4,237,441, 965,344 filed 12-1-78 (Middleman et al) now U.S. Pat. No. 4,238,812, 965,345 filed 12-1-78 (Middleman et al), now U.S. Pat. No. 4,242,573, and 75,413 filed 9-14-79 (Van Konynenburg), now U.S. Pat. No. 4,304,987, and the eight applications filed Apr. 21, 1980 by Gotcher et al Ser. No. 141,984, now abandoned (MP0712, 157/111), Middleman et al, Ser. No. 141,987, now U.S. Pat. No. 4,413,301, (MP0713, 157/112), Fouts et al, Ser. No. 141,988, now abandoned (MP0714, 157/113), Evans, Ser. No. 141,989 (MP0715, 157/114), Walty, Ser. No. 141,990, now U.S. Pat. No. 4,314,231, (MP0719, 157/161), Fouts et al, Ser. No. 141,991, now U.S. Pat. No. 4,545,926, (MP0720, 157/162), Middleman et al, Ser. No. 142,053, now U.S. Pat. No. 4,352,083, (MP0724, 157/167) and Middleman et al, Ser. No. 142,054, now U.S. Pat. No. 4,317,027, (MP0725, 157/168). The disclosure of each of these patents and applications is incorporated herein by reference.
The known self-limiting strip heaters are not satisfactory for use in situations in which they may be externally heated to temperatures substantially higher than the temperatures which they reach during their normal use as heaters, as for example during intermittent steam-cleaning of pipes which are heated by the heater during normal operation. When exposed to such temperatures, known heaters, whether powered or not during the exposure, increase in resistivity at a rate which rapidly renders them ineffective.